ELECTROCARDIOGRAMS 275 



It will be seen from this that the excitation wave travels with great 

 speed over a semicircular pathway which has a clockwise direction in 

 the right ventricle and an anti-clockwise direction in the left (Fig. 84-5). 

 From this it passes at a more leisurely rate through the ventricular 

 muscle reaching points upon the pericardial surface at intervals which 

 to a great extent are controlled directly by the relative thicknesses of 

 the heart wall at the different points. It is to be expected that the 

 spread of negativity through the ventricular muscle will set up action 

 currents which will vary in their direction in accordance with the change 

 in direction of the excitation wave. The alterations in the course of 

 the electromotive changes will be in the nature of a rotation which will 

 have a clockwise direction in the right and an anti-clockwise direction 

 in the left chamber. Lewis, from calculations based upon Einthoven's 

 triangle method, determined the direction of rotation of the electrical 

 axis separately in each ventricle after one or other division of the bundle 

 had been severed so as to isolate that chamber effectually, insofar as 

 the excitatory process was concerned, from its companion. It was 

 found that the directions of the electrical axes calculated during the 

 production of the Q.R.S. complex were, at the same instants of time, 

 closely in accord with the direction of the excitation wave as determined 

 from observations made with direct contacts placed upon the heart. 

 From this it has been concluded that the direction of the electrical axis 

 corresponds to the average direction in which the excitation wave is 

 tending to move at a given moment. 



Interruption of the conducting pathway in one ventricle does not affect 

 the course of the excitation wave in the contralateral ventricle during 

 the time that the initial deflections Q.R.S. of the electrocardiogram are 

 being inscribed but since there is no spread across the septum from one 

 ventricle to the other, the spread through the ventricle on the operated 

 side is prevented during this period. 



An electrocardiogram taken from an animal of which the right 

 or left conducting branch had been severed, was therefore a rep- 

 resentation, insofar as the initial deflections were concerned, of the 

 electrical changes occurring in the contralateral ventricle. That is 

 to say, a record taken after division of the right branch gave a 

 picture of the electrical changes occurring in the left chamber alone, 

 whilst one taken after division of the left branch was a rep- 

 resentation of electrical changes in the right chamber. For the sake 

 of convenience in the description of his researches, Lewis has termed 

 such records of one or other ventricle a levocardiogram or a dextrocardio- 

 gram respectively. Since the physiological electrocardiogram is a represen- 

 tation of the potential differences occurring in the heart as a whole, it has 

 been termed appropriately the "bicar diagram. In the further study of 



